1. Field of the Invention
The present invention relates to the recording and reproducing of data on a disk recording medium such as a magnetic disk or an optical disk, and more particularly, to a driving control of a spindle motor for rotating the recording medium.
2. Description of the Background Art
Conventionally, in an apparatus for recording data by using a disk recording medium such as a hard disk drive, it is customary to drive the spindle motor for rotating the disk by switching driving currents, and to carry out the servo control for maintaining a constant rotational speed of the spindle motor by detecting an error with respect to a target rotational speed and changing a driving waveform of the motor, i.e., a pulse width or an amplitude of a driving current waveform of the motor, according to the detected error. By means of this servo control, it is possible to drive the spindle motor at the rotation precision satisfying the required device specification.
However, in this type of a conventional spindle motor driving mechanism, while it is possible to suppress the variation of the rotational speed around the axis of the spindle motor sufficiently by means of the above described servo control, it is impossible to suppress the vibration such as the so called axial vibration occurring in a direction other than that around the axis of the spindle motor, and the servo control for stabilizing the rotational speed actually has a side-effect to promote the vibration of the spindle motor and the generation of noises.
This vibration of the spindle motor has an adverse effect especially on the tracking characteristic of a magnetic head in the magnetic disk device realizing a high recording density due to a high track density, and in addition, it also causes the spike noises which lower the C/N (Carrier to Noise) ratio of the reproduced signals.
This vibration of the spindle motor can be reduced by improving the mechanical precision of the motor components, by improving the manufacturing precision of the motor or the bearing for example, but it is impossible to remove this vibration of the spindle motor completely by the improvement of the mechanical precision alone.
Moreover, even if it is possible to reduce this vibration of the spindle motor by the improved mechanical precision initially, this vibration of the spindle motor would increase as time elapses because of the deterioration of the mechanical precision such as the wear of the bearing. In addition, the improvement of the mechanical precision further than what is required by the device specification, solely for the purpose of reducing this vibration of the spindle motor, would require a considerable increase of the device cost which is commercially quite disadvantageous.